Because of the great climatic impacts of ENSO has on Northern America and the globe, assessing and understanding ENSO and its changes in response to global warming is essential to addressing issues relating to the interaction of climate change and natural climate variability. Instrumental records, albeit short, bear witness to the rich diversity and complexity of ENSO in terms of significant variations in its amplitude, frequency, and location of peak sea surface temperature anomalies, warm/cold asymmetry, synchronization to the annual cycle. Climate models exhibit large spread in simulating these basic properties of ENSO. Our current physical understanding of ENSO diversity in observation and simulated in climate models is still severely limited. Our DOE project has led to some important progresses in addressing this issue. For examples: (1) we made progresses in advancing our understanding of some key physical processes that contribute to changes in in ENSO stability and occurrence frequency of the El Nino extreme events; (2) we developed a simple method and matric for estimating the state-dependency of the stochastic forcing of ENSO and a simple theory relating it to the occurrence frequency of the El Nino extreme events; (3) we uncovered a combination mode of ENSO and annual cycle, which may enrich ENSO temporal variability. We will present the highlights of these findings and discussion the directions of future research.